The ability of monoclonal antibodies (MAbs) to target and accumulate in tumors has been amply demonstrated in both animal models and man. Although the specificity of this targeting varies with different MAbs, the amount of antibody that binds tumor, relative to the amount that binds normal tissue has been high enough to permit clear tumor images using appropriate radioactive labels.
For therapy, however, the quantity of antibody that accumulates at the tumor site determines the payload of therapeutic radionuclide, toxin, or drug delivered to the tumor. Early studies measuring the percent injected dose found in tumors in patients after injection with radiolabeled MAbs have shown extremely low values on the order of 0.01-0.1%. (See, e.g., Goldenberg, D. M., Arch. Pathol. Lab. Med. 112: 580-587 (1988); Epenetos et al., Cancer Res. 46: 3183-3191 (1986)). Considering the relative resistance of most malignant solid tumors to drugs and radiotherapy, it is imperative that the accumulation of MAbs at the tumor site be substantially improved to obtain an adequate therapeutic index required for maximum tumor destruction and sustained therapy.
In order to improve the effectiveness of monoclonal antibody (MAb) therapy, a number of investigators have produced immunoconjugates composed of MAbs and biological response modifiers, such as cobra venom factor (Vogel, C. and Muller-Eberhard, H., Proc. Natl. Acad. Sci., USA, 78(12): 7707-7711 (1981), Vogel, C. et al., "Hematology and Blood Transfusion," in Modern Trends in Human Leukemia VI, 29: 514-517 (1985), Rolf Neth, Ed.), formyl-methionyl-leucyl-phenylalanine (Obrist, R. Sandberg, A., Cellular Immunology 81: 169-174 (1983); Obrist, et al., Bent 53: 251 (1986)), and interferon-.gamma. (Flannery, G. et al., Eur. J. Cancer Clin. Oncol., 20(6): 791-798 (1984)). These studies demonstrated that immunoconjugates could direct specific responses, like tumoricidal effects or chemotaxis, specifically to the tumor site without demonstrable toxicity in normal organs and tissues. However, this approach to enhancing the effectiveness of monoclonal antibody therapy did not solve the problem that only extremely low levels of monoclonal antibody accumulate at the tumor site.
Another approach to this problem is to alter the physiology of tumor vessels to enhance the tumor uptake of macromolecules. This approach used MAbs as carriers for the delivery of vasoactive peptides and compounds to the tumor. Seven different vasoactive compounds, namely tumor necrosis factor .alpha., interleukin-1.beta., interleukin-2 (IL-2), physalaemin, histamine, bradykinin, or leukotriene, were chemically linked to a monoclonal antibody that targets degenerating cells in necrotic regions of tumors. While all of seven immunoconjugates showed specific enhancement of monoclonal antibody uptake in tumors, the IL-2/MAb conjugate gave the highest percent injected dose per gram of tumor. (Khawli, et al., Cancer 73: 824-831 (1994))
Interleukin-2 is a promising candidate for efforts to improve the therapeutic index of MAb therapy. It is a 15,000 Dalton protein produced by helper T lymphocytes. As a potent biological modulator of the immune system of animals and man, it occupies a central role in the augmentation of cell-mediated immune responses. Its major functions include the proliferation of T lymphocytes (Morgan, D. A, et al., Science 193: 1007-1008, (1976)) and the generation of non-specific tumor killing by activated macrophages, lymphokine-activated killer cells (LAK cells) (Grimm, E. A., et al., J. Exp. Med. 155: 1823-1841(1982)), and tumor infiltrating lymphocytes (TIL cells)(Rosenberg, S. A., et al., Science 233: 1318-1321(1986)). In addition to its cytokine activity, IL-2 has been shown to produce vascular permeability when administered systemically by causing the efflux of intravascular fluids to the extravascular spaces (capillary leak syndrome)(Rosenstein, M., et al., Immunology 137: 1735 1742 (1986); Ohkubo, C., et al., Cancer Res. 51: 1561 1563 (1991); Edwards, M. J., et al., Cancer Res. 52: 3425-3431(1992); Damle, N. K., et al., J. Immunol. 142: 2660-2669 (1989)).
Human IL-2 is a globular protein consisting of 133 amino acids and is similar in structure to Interleukin-4 and Granulocyte/Macrophage-Colony Stimulating Factor (GM-CSF)(Bazan, J. F., Science 257: 410-412 (1992)). Structural studies of IL-2 show that it is composed of four major amphipathic alpha helices arranged in an antiparallel fashion, with the hydrophobic faces making a very stable hydrophobic core (Bazan, J. F.,(1992); McKay, D. B., Science 257: 412-413 (1992)). In addition, one disulfide bond is important to stability of the tertiary structure and is essential for the biologic activity of IL-2 (Landgraf, B. E., Proteins 9: 207 (1991)). Loss of this disulfide bond, as well as even minor changes in the primary or secondary structure abrogate IL-2 cytokine activity as shown by site-directed mutagenesis studies (Cohen et al., Science 234: 349-352 (1986)). Previous studies have shown that the intact, native IL-2 structure is a prerequisite for biologic activity because of the unique structure of the IL-2 receptor, which may be low affinity (.alpha. chain), intermediate affinity (.beta. and .gamma. chains), or high affinity (.alpha., .beta., and .gamma. chains)(Smith, K. A., Blood 81: 1414-1423(1993)).
When IL-2 is used alone as a therapeutic agent or in combination with other agents, such as interferon-.alpha., LAK, TILS, or monoclonal antibodies, 20-50% partial and complete responses are obtained in certain human neoplasms, including lymphoma, renal cell cancer, and melanoma (Lotze, M. T., "Interleukin-2," in Human Cytokines, Ed. by Aggarwal and Gutterman, pp. 81-96 (1992); Marincola, F. M., Biologic Therapy of Cancer Updates 4(3): 1-16 (1994); Thompson, J. A., et al., Hematologic Growth Factors 2(5): 351-355 (1994)). IL-2's activity against cancer has been ascribed to its ability to mediate enhanced host immune resistance, primarily through T-cell expansion and directing the traffic into tissues of such activated T-cells. However, the administration of IL-2 causes several systemic effects tied to the capillary leak syndrome, including edema formation, hypotension, and renal dysfunction. These side effects limit the administration of higher dosages of IL-2 and can lead to discontinuation of the therapy.
One approach to reducing the toxic effects of systemic IL-2 administration would be to target IL-2 to a tumor site using an antibody delivery system. Consequently, IL-2 has been successfully incorporated into a number of immunoconjugates and fusion proteins. A number of investigators have demonstrated that IL-2 cytokine activity can be preserved in such constructs. For example, Gillies et al. (Proc. Natl. Acad. Sci., USA 89, 1428-1432 (1992)) assembled a genetically engineered fusion protein consisting of a chimeric anti-ganglioside GD2 antibody and IL-2, which could enhance the killing of GD2-expressing melanoma target cells by a TIL cell line. Similarly, Savage et al. (Br. J. Cancer 67: 304-310 (1993)) constructed a single chain antibody IL-2 fusion protein that retained the ability to bind antigen as well as low affinity IL-2 receptors and to stimulate the proliferation of human peripheral blood lymphocytes. Moreover, Naramura et al. (Immunol. Lett. 39: 91-99 (1994)) demonstrated that a genetically engineered fusion protein, comprised of IL-2 and a mouse/human chimeric monoclonal antibody directed against human epidermal growth factor, activated immune effector cells in vitro and enhanced cellular cytotoxicity against human melanoma cells.
In contrast to work capitalizing on IL-2's cytokine activities, another approach focussed on harnessing its toxicity. For example, IL-2 has been covalently linked to a tumor-specific monoclonal antibody (MAb/IL-2) to induce localized vasopermeability at the tumor site (Khawli, et al.,(1994); LeBerthon et al., Cancer Res. 51: 2694-2698 (1991)). The generation of leaky tumor endothelium by pretreatment with MAb/IL-2 produced a 3-4 fold increase in monoclonal antibody uptake, which was not observed in normal tissues. Unlike the previous studies cited above (Gillies et al., Savage et al., and Naramura et al.), the chemistry used to link the IL-2 to monoclonal antibodies destroyed the cytokine activity of IL-2 without affecting its vasopermeability effects.
Taken together, these studies emphasize the finding that the vasopermeability activity of IL-2 appears to be a stable property of the molecule compared to the cytokine activity, which appears to be more sensitive to perturbations in the tertiary structure of IL-2. Consequently, it would be advantageous to develop a synthetic IL-2 peptide that retains the biologic activity of vasopermeability, but need not retain the cytokine activity of the molecule. Such a peptide may be used to generate potent vasoactive immunoconjugates, having reduced toxicity for normal tissues, that can be used to enhance the delivery of therapeutic and diagnostic agents in tumors and other tissues.